You wouldn't really be able to track a mobile robot with sophisticated evasion skills. It could popup randomly, kill with unmatched precision, then disappear. An army of these robots would be nearly unstoppable.
Several robots are being armed these days, with grenade launchers, sniper rifles, and both light and heavy machine guns (as seen on the guard sentries in the article) https://www.qinetiq-na.com/products/unmanned-systems/maars/
The only real limitation to current robots is the power-source / batteries. Right now we have to either strap a gasoline engine on it, tether it, or create it so that it's efficient enough to run off of existing batteries, which greatly limits their abilities.